1. Field of the Invention
The present invention relates to fiber optic devices used for communication and other applications. More specifically, the present invention relates to electro-optical modulators.
2. Description of the Related Art
Optical fibers are widely used for communication and other applications. Inasmuch as optical fibers offer high bandwidth at low cost, they facilitate the communication of large amounts of data inexpensively. Currently, optical systems must inevitably interface to electrical systems at the inputs and outputs thereof. Over the years, many devices have been developed to facilitate the electrical to optical transition and vice versa. For example, electro-optical modulators have been developed to effect a conversion of electrical signals to optical signals.
Presently, much effort in the art is focused at the high frequencies to further increase system bandwidth and capacity. At very high frequencies ( greater than 10 gigahertz (GHz)), traveling wave electro-optical modulators are often employed. In certain traveling wave modulators, a microstrip is disposed on an optical fiber. This allows an electrical waveform to travel in an electrical channel in the microstrip along with an optical waveform that travels in an optical channel in the optical waveguide. A particularly advantageous traveling wave modulator is known in the art as a Mach-Zehnder configuration.
As discussed in detail below, in a conventional Mach-Zehnder structure, data (electrical signals) comes in through a microstrip transmission line. It is then split into two electrodes; each one positioned directly on top of an optical waveguide. The electrical field, encoded with data, changes the index of refraction through an electro-optic effect in the optical waveguide below. In one implementation the dipoles in the two waveguides are arranged in opposite orientations during the fabrication process. Consequently, by applying same electrical signal to both arms, their refractive indices are changed in opposite directions. In one arm the refractive index increases, while in the other arm it decreases.
In another implementation the dipoles in the two wave-guides are arranged in the same orientations during the fabrication process. In this case the modulating signal in the two arms has complementary polarities. Therefore, also in this case the refractive indices are changed in the opposite directions.
In both implementations a constant intensity laser beam is split into the two optical waveguides whose refractive indices are modulated. In one arm the light speeds up while in the other the light slows down. This creates a phase differential between the two optical signals. Consequently, when these two light beams are brought together, they interfere with each other and the combined intensity is amplitude modulated with the input data. Thus, an electrical modulation is converted to a phase modulation and the phase modulation results in an amplitude modulation of the output beam.
In practice, Mach-Zehnder operation is set up by configuring the device so that when no input signal is applied, the output light is at one of three levels: a) minimum, b) maximum, or c) a so-called xe2x80x98quadrature pointxe2x80x99. The minimum output light operating point is achieved when the light coming from the two arms of the modulator are combined with phase difference of 180 degrees. This operating point provides maximum signal-on to signal-off ratio. The maximum output light operating point is achieved when the phase difference is zero and the quadrature operating point is achieved when the phase difference is 90 degrees. At this operating point maximum signal linearity is achieved and therefore, the quadrature point is selected when the signal linearity is important.
In all case, Mach-Zehnder device configuration requires an effective adjustment of the optical length of one arm relative to the other on the order of a fraction of a wavelength, i.e., on the order of a micron or less. Inasmuch as this is impractical in the physical domain, it is generally effected electrically via a direct current (DC) bias adjustment. The bias voltage is applied to one or both arms in such a manner as to create the specified optical path length differential between the two arms. Two methods are currently predominantly used in the art to establish a desired DC bias for Mach-Zehnder type traveling wave electro-optical modulators.
As discussed more fully below, in accordance with a first method, a DC bias voltage (Vb) is combined with a modulating signal (Vm) by means of two capacitors (C) and an inductor (L). The purpose of the capacitors is to prevent very large DC current flow through the terminating resistors Ro and the purpose of the inductor is to avoid a shorting of the modulating signal (Vm) through a low impedance of a DC power supply, Vb.
The problem with this approach is that the large capacitors and inductor required have relatively high parasitic components, which increase the impedance of the capacitors, reduces the impedance of the inductor and creates undesired resonances. Therefore, this arrangement is used primarily when the modulating signal (Vm) is limited to relatively low frequencies (F less than 10 GHz).
In accordance with a second conventional method for biasing Mach-Zehnder modulators, the modulator includes two separate sections. One section is an RF modulator as in the first method. A second separate section provides for a bias setting. This approach, however, while avoiding the problems of the first approach, introduces another drawback. Namely the total optical path that the optical beam travels in the polymer is substantially longer and therefore the insertion losses are also substantially higher than in the first approach.
Hence, a need remains in the art for an improved system or method for biasing traveling wave electro-optical modulators operating at high frequencies. Specifically, a need remains in the art for an improved system or method for biasing Mach-Zehnder traveling wave electro-optical modulators which offers minimal parasitic inductive and capacitive effects and minimal insertion loss while operating at high frequencies.
The need in the art is addressed by the electro-optical modulator and the method for biasing a Mach-Zehnder modulator of the present invention. The inventive modulator includes a layer of material at least partially transparent to electro-magnetic energy. A first conductive layer is disposed on a first surface of the layer of at least partially transparent material. A second conductive layer is disposed on a second surface of the layer of at least partially transparent material. A layer of insulating material is disposed on the second conductive layer and a third conductive layer is disposed on the layer of insulating material. In the illustrative application, the modulator is a Mach-Zehnder modulator. A biasing potential is applied to the second conductive layer of the modulator and a modulating voltage is applied across the first and the third conductive layers.